|Publication number||US6906861 B2|
|Application number||US 10/415,640|
|Publication date||Jun 14, 2005|
|Filing date||Nov 2, 2001|
|Priority date||Nov 3, 2000|
|Also published as||CN1200289C, CN1478205A, DE10054503A1, DE10054503B4, DE50105339D1, EP1356319A2, EP1356319B1, US20040021945, WO2002037145A2, WO2002037145A3, WO2002037145A9|
|Publication number||10415640, 415640, PCT/2001/12679, PCT/EP/1/012679, PCT/EP/1/12679, PCT/EP/2001/012679, PCT/EP/2001/12679, PCT/EP1/012679, PCT/EP1/12679, PCT/EP1012679, PCT/EP112679, PCT/EP2001/012679, PCT/EP2001/12679, PCT/EP2001012679, PCT/EP200112679, US 6906861 B2, US 6906861B2, US-B2-6906861, US6906861 B2, US6906861B2|
|Inventors||Wayne Robert Tompkin, René Staub, Andreas Schilling|
|Original Assignee||Ovd Kinegram Ag|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (18), Referenced by (20), Classifications (9), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims priority based on an International Application filed under the Patent Cooperation Treaty, PCT/EP01/12679, filed on Nov. 2, 2001, and German Patent Application No. 100 54 503.3, filed on Nov. 3, 2000.
The invention relates to a light-diffractive binary grating structure and the use thereof in a security element as set forth in the classifying portions of claims 1 and 10.
Such grating structures are distinguished by a high level of diffraction deficiency and pronounced asymmetry and polarisation of the diffracted light and are suitable for the surface patterns which are known as visual security features, comprising a mosaic of surface elements occupied by diffractive structures.
Those security features serve as an authenticity feature which is difficult to forge, for valuable articles, documents, identity cards or passes, bank notes and so forth and are described for example in EP 0 105 099 B1. The surface elements of the security features have periodic diffractive grating structures with predetermined grating parameters such as profile, spatial frequency and azimuthal orientation and so forth, in such a way that the aspect of the illuminated security feature changes in a predetermined fashion in respect of color and pattern, upon being rotated and/or tilted. If oppositely oriented asymmetric grating structures are arranged in adjacent surface elements, as is known for example from EP 0 360 969 A1, the levels of intensity of the surface elements, such levels being perceived by a viewer, are interchanged when the substrate is turned through 180°.
DE 32 06 062 A1 shows a simple binary grating structure and the use thereof for a security feature. The binary grating structure has as its profile a structure which follows a simple periodic rectangular function. The pulse duty cycle of the rectangular function, that is to say the ratio of the width of the valley or trough to the length of the period of the rectangular structure, is constant and is of a value of 50% so that the diffracted light has the greatest degree of color saturation.
A development of the security feature with binary grating structures is described in EP 0 712 012 A1. A surface element of the security feature is occupied by a periodic rectangular structure. The pulse duty cycle of the rectangular function continuously changes over the surface element from a maximum value to a minimum value so that, when the surface element is illuminated, the degree of color saturation reaches a maximum in a region within the surface element. Also shown are non-periodic rectangular structures which are arranged with randomly selected dimensions in respect of depth and width to form a diffraction structure in order to enhance the anti-forgery aspect of the security elements. It will be noted however that the resulting pattern is highly complex and is difficult for an observer to identify as the genuine security feature.
U.S. Pat. No. 4,846,552 discloses an electron beam lithography process for producing binary grating structures for focusing optical elements. Even with the simple rectangular structures with a pulse duty cycle of 50%, it is possible to achieve high levels of diffraction efficiency for linearly polarised light by a clever choice of the depth of the structure, for example 25% of the period length of the rectangular structure. The importance of the binary grating structures is based on the fact that they can be produced with a high degree of accuracy and replicated by molding.
The above-mentioned simple rectangular structures can be copied using holographic means, in which respect, in the case of a copy of the symmetrical rectangular structure with a pulse duty cycle of about 50%, there is only a slight drop in the level of diffraction efficiency. The level of diffraction efficiency of the copy worsens, the more asymmetrical that the rectangular structure is. A surface element with the rectangular structure whose pulse duty cycle continuously changes along a predetermined direction in accordance with EP 0 712 012 A1 has in the copy, instead of the changing color saturation, a variation in the intensity of the diffracted color. As changing color saturation is visually difficult to distinguish from changing color intensity without reference to a genuine security element, an unauthorised copy of the security element cannot be immediately recognised.
The object of the present invention is to provide a grating structure which is inexpensive to replicate and which is suitable for use in optically diffractive security elements, which is expensive and complicated to produce and difficult to copy.
In accordance with the invention the specified object is attained by the features recited in the characterising portions of claims 1 and 10. Advantageous configurations of the invention are set forth in the appendant claims.
Embodiments of the invention are described in greater detail hereinafter and illustrated in the drawing in which:
A portion shown in
The periodic binary grating structure diffracts electromagnetic radiation. For security features which are to be adjudged visually the length of the period T and the profile height h of the grating structure is to be adapted to the wavelength λ of the visible light whose spectrum of the wavelengths λ extends in the range 380 nm<λ<780 nm. The period T is for example of a value from the range for T of 250 nm to 5000 nm. For ‘zero order’ microstructures the condition λ/T≧1 applies. With the ‘zero order’ microstructures all diffraction orders apart from the zero order are suppressed. Under the condition λ/T<1 in contrast the light incident on the binary grating structure is also diffracted into higher diffraction orders. Those microstructures are not to be copied with conventional holographic methods if the period T≦2 μm.
The profile height h of the valleys 4 is constant. The microscopic mesa structure 2 is optically effective and has valleys 4 and plateaux 5 which are parallel, straight, of a meander shape or curved. The mesa structure 2, as a periodic grating structure, is described by its parameters, an azimuth angle ψ relative to a significant direction, the period T or spatial frequency f=1/T, a profile shape of the mesa structure 2 and a profile height h. The geometrical profile height hG within the mesa structure 2 is not to be confused with the optically effective profile height h. If a transparent material with a refractive index n>1 fills up the valleys 4, then the geometrical profile height hG multiplied by the refractive index n becomes optically effective as the profile height h. The optically effective profile height h is selected from the range of 25 nm to 5000 nm, in which respect preferred values of the profile height are for example in the lower range up to a quarter of the wavelength, λ/4. If the substrate 1 illustrated here has the negative profile shape of the mesa structure 2, the substrate 1 is suitable for molding of the mesa structure 2 in plastic material or for stamping or embossing very soft metal (Ag, Al, Au, Cu and so forth). That permits inexpensive replication of the mesa structure 2. Harder metals such as steel, copper-nickel alloy for coins and so forth are to be structured by a per se known, metal-removing method.
The transparent material is optically clear and transmits the entire spectrum of visible light (=material which is as clear as glass) or only certain spectral ranges thereof (=colored material).
The laminate 8 in
In the embodiment at the left in
Governed by manufacturing considerations the cross-sections of the valleys 4 have slight deviations from the ideal rectangular shape.
The mesa structure 2 in
The ratio of the pulse width t of the rectangular pulses R to the period length T is referred to as the pulse duty cycle t/T. At least one rectangular function R has a pulse duty cycle of at most 6%, for example the rectangular function R3(x″) in the view in
The phase shifts φ1, φ2, φ3, . . . and the pulse widths t1, t2, t3 . . . are such that on the one hand in the resulting rectangular function M(x) of the mesa structure 2 only a single valley 4 between two plateaux 5 is of a greatest width w greater than a seventh of the period T. On the other hand at least one valley 4 measures a smallest width w of at most a fifth of the wavelength λ. Parameters such as the number N, the period T, the profile height h, the phase shift φ and the pulse width t determine the mesa structure 2.
Due to manufacturing considerations the structure width of the mesa structure 2 is not to be less than a smallest width. The smallest structure width denotes the smallest of the values of the widths w and pulse widths t arranged in the mesa structure 2. The smallest structure width which can be managed from manufacturing aspects depends on the geometrical profile height hG as molding of the smallest structure width of more than half the geometrical profile height hG causes major manufacturing molding difficulties. The smallest structure width which can be attained technically today is about 50 nm with a geometrical profile height hG of up to 100 nm.
Due to the above-mentioned conditions the number N of the rectangular functions R(x) forming the function M(x) is limited upwardly by the period length T and the smallest structure width to about 10, wherein N is preferably in the range of 3 up to and including 5.
In an embodiment of the mesa structure with the period T on the one hand the sum of the pulse widths t of all plateaux 5 in the one half-period T/2 is greater than the sum of the widths w of all valleys 4 in the same half-period T/2 and on the other hand in the second half-period T/2 the sum of the pulse widths t of all plateaux 5 is smaller than the sum of the widths w of all valleys 4 of the second half-period T/2. Each half-period T/2 includes at least one valley 4 and one plateaux 5. Such a mesa structure 2 has an asymmetrical diffraction behaviour and deflects the incident light asymmetrically into the various diffraction orders. Associated with the asymmetrical mesa structure 2 is a significant direction (grating vector) which is marked by an arrow 19 (
The above-described mesa structures 2 have microscopic and sub-microscopic structures, that is to say structure widths smaller than the wavelength λ of light. For such structures the scalar theory can no longer qualitatively describe the diffraction behaviour of the sub-microscopic mesa structure 2. Only an application of exact electromagnetic theory and precise calculations as are contained for example in the book ‘Electromagnetic Theory of Gratings’ by R Petit, Editor, Springer-Verlag, Heidelberg 1980, can with a great deal of computing expenditure describe the diffraction behaviour of the mesa structures 2 with sub-microscopic structures. Scalar theory fails because the behaviour of light is completely different for the TE- and TM-polarised electromagnetic waves upon incidence on the binary grating structure.
In the case of TE-polarisation in which the electrical field is oriented parallel to the valleys 4 of the mesa structure 2, such strong surface currents flow in the reflection layer 11 (
The light-diffractive binary grating structures which as a profile have the asymmetrical mesa structures 2 with the sub-microscopic structure elements exhibit extraordinary diffraction properties upon being illuminated with light which is incident perpendicularly on to the mesa structures 2. The incident light is diffracted primarily into a single diffraction order while the diffracted light in the other diffraction orders involves only low levels of intensity which are scarcely visually perceptible.
The operation of checking the influence of the polarisation capability of the grating structure is effected in that, as shown in
The binary grating structure in
Pulse width t
Phase shift φ
The profile height hG is 90 nm and the smallest structure width is 75 nm, wherein the smallest structure width occurs both as a pulse width t=75 nm in the case of the rectangular function R1 and also as the width w=75 nm of the valley 4 (
The mesa structure 2 shown in
The profile height hG is 150 nm, the smallest structure width being 100 nm. The smallest structure width occurs both as a pulse width t=100 nm in the case of the rectangular functions R1(x), R3(x) and R4(x) and also as the width w=100 nm of the valley 4 (
Pulse width t
Phase shift φ
In the case of perpendicularly incident light the mesa structure 2 shown in
If unpolarised light 22 (
The ratio of the surface brightnesses in the surface element 21 of
The binary grating structure in
Pulse width t
Phase shift φ
The profile height hG is 90 nm and the smallest structure width is 55 nm, wherein the smallest structure width occurs as the width w=55 nm of the valley 4 (
The mesa structure 2 shown in
If unpolarised light 22 (
Therefore the contrast disappears at the common boundary 27 of the two elements 25, 26 in both azimuthal orientations 0° and 180° and the internal element 26 cannot be recognised in the surface of the background element 25.
In an embodiment of the surface element 21 the internal elements 26 are in the form of alphanumeric characters, for example a text, or in the form of a bar code, which are visible in the polarised light on the background element 25 but which cannot be seen in usual unpolarised daylight.
The diffraction structures which are used in the combinations shown in
In an embodiment of
As is shown by means of the example of
In an embodiment the structures of the raster surfaces 31 are those of a Fourier hologram or kinoform containing information. The information can be rendered visible on a screen and read, only with monochromatic illumination of the surface element 25 having those raster surfaces 31.
The diffraction structures of the type of the above-described mesa structures 2 (
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|U.S. Classification||359/567, 428/916, 359/575, 359/572|
|Cooperative Classification||B42D25/328, Y10S428/916, G02B5/18|
|Aug 25, 2003||AS||Assignment|
Owner name: OVD KINEGRAM AG, SWITZERLAND
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:TOMPKIN, WAYNE ROBERT;STAUB, RENE;SCHILLING, ANDREAS;REEL/FRAME:014449/0799;SIGNING DATES FROM 20030428 TO 20030429
|Dec 13, 2005||CC||Certificate of correction|
|Dec 1, 2008||FPAY||Fee payment|
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